_read_regular_file O_NOFOLLOW

Trap: O_NOFOLLOW looks like complete symlink rejection, but it protects only the final component; symlinked parent directories remain traversable, while getattr(os, "O_NOFOLLOW", 0) silently disables protection when unavailable. Fix: traverse components relative to bound directory descriptors with no-follow checks, reject symlinked parents, and fail before opening when the required primitive is unavailable.